In general, an ethylene polymer is widely used as a resin material for various molded products. The ethylene polymer must satisfy different requirements depending on its molding method and purpose. For example, an ethylene polymer having a relatively low molecular weight and a narrow molecular weight distribution is suitable for injection molding. On the other hand, an ethylene polymer having a relatively high molecular weight and a wide molecular weight distribution is suitable for blow molding or extrusion. It has heretofore been known that the use of a so-called Phillips catalyst comprising chromium trioxide supported on an inorganic oxide such as silica can provide an ethylene polymer having a wide molecular weight distribution suitable for blow molding. Further, a single-stage or multi-stage polymerization process using a Ziegler catalyst disclosed in JP-A-2-123108 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-4-18407 and JP-A-5-230136 can provide an ethylene polymer having a wide molecular weight distribution suitable for blow molding and extrusion.
However, it has recently been desired to further enhance the quality of ethylene polymers suitable for blow molding and extrusion. A molded product obtained by blow-molding or extruding an ethylene polymer having a wide molecular weight distribution produced in the presence of the foregoing catalyst leaves something to be desired in some respects. (1) Referring to moldability, since the ethylene polymer has an insufficient melt tension and die swell, the resulting blow-molded product has an uneven thickness. Further, the ethylene polymer has a poor bubble stability during extrusion. (2) Since the molded product has a roughened surface, it is undesirable from the standpoint of external appearance.
A process for the production of an ethylene polymer having a wide molecular weight distribution using a catalyst made of a chromium compound and an alumoxane has recently been disclosed in JP-W-A-7-503739 (The term "JP-W-A" as used herein means an "published Japanese national stage of international application"). However, the ethylene polymer obtained by this process leaves something to be desired both in moldability and surface properties of molded product.
Further, JP-A-2-105806 and JP-A-2-185506 disclose a process for the production of an ethylene polymer having a wide molecular weight distribution suitable for blow molding and extrusion in the presence of a catalyst made of a solid catalyst component comprising chromium trioxide supported on an inorganic oxide carrier, an alumoxane, an organic aluminum alkoxide or an organic aluminum siloxide. However, the ethylene polymer obtained by this process, too, leaves something to be desired both in moldability and surface properties of molded product.
Moreover, JP-A-54-120290 discloses a process for the production of an ethylene polymer having a wide molecular weight distribution suitable for blow molding and extrusion in the presence of a catalyst made of a solid component obtained by treating a chromic acid ester supported on silica with an organic aluminum alkoxide and an alumoxane. However, the ethylene polymer obtained by this process, too, leaves something to be desired both in moldability and surface properties of molded product.
An object of the present invention is to give solution to the foregoing problems and hence produce efficiently an ethylene polymer having an excellent moldability which gives a molded product free of roughened surface.